From the aforementioned copending applications and other literature, it will be apparent that considerable attention has been directed in recent years, because of the need to reduce environmental pollution, to the more effective cleaning of flue gases before they are discharged into the atmosphere.
The flue gases with which the invention is concerned are primarily the sulfur-containing flue gases resulting from the combustion of fossil-fuels, e.g. coal, in boilers and especially power plant boilers.
Generally speaking, the sulfur content of such flue gases is in the form of sulfur dioxide and sulfur trioxide, so that the scrubbing of the flue gases can be effected with a scrubbing solution in an upright scrubbing column, the scrubbing liquid containing a calcium compound which reacts to form compounds with the sulfur oxides, namely calcium sulfate and calcium sulfite.
Since calcium sulfate recovered as described in the copending applications may be a valuable by-product of the gas-cleaning process because of its utility as gypsum in the construction trades, provision may be made for oxidizing calcium sulfite to the calcium sulfate.
The calcium compound which is used may be any compound which is soluble in the scrubbing liquid or which forms a suspension thereof and which is capable of enabling the calcium to react with the sulfur dioxide and sulfur trioxide or the acids formed thereby during the scrubbing action. The most common calcium compounds for this purpose are the calcium carbonate which can be suspended in the scrubbing liquid, calcium oxide which can be added thereto or calcium hydroxide which can be utilized, for example as milk of lime, as an additive to the scrubbing liquid. For convenience of description, all of these additives, because they act similarly, may be referred to as lime hereinafter.
More specifically, it is known to provide a scrubber for the desulfurization of a fossil-fuel boiler flue gas, especially an electricity-generating power plant boiler flue gas, which has the configuration of a tower, i.e. is generally cylindrical and upright and is provided with a flue gas inlet at the bottom of this tower but above the sump of the column, a flue gas outlet in the region of the top or head of the tower, a device for feeding the scrubbing liquid, and the aforementioned sump in which the scrubbing liquid collects.
From this sump, a suspension of the scrubbing liquid, the calcium compounds (lime) and previously precipitated calcium sulfite and/or calcium sulfate can be recirculated to the tower thereabove to scrub the rising flue gas in the counterflow scrubbing action.
From the sump, moreover, the suspension can be withdrawn as desired for recovery of calcium sulfate. At the head of the tower, a liquid separator (water separator) can be provided to strip entraining scrubbing liquid from the departing gas and to return the recovered scrubbing liquid to the tower and the sump.
From this liquid separator, the scrubbed gas may be discharged through a flue gas outlet which can communicate radially with the shell of the separator.
The separators provided for such scrubbers heretofore were generally of the cyclonic or vortex type. With high velocities of the flue gas in the head of the tower, this has created problems because water recovered by the separator tended to be reentrained by the gas. Furthermore, there was a tendency for deposits of calcium sulfite and/or calcium sulfate to build up upon the surfaces of the separator.